1. Field of the Invention
The invention is directed to a circuit and a method for signal voltage transmission within a driver of a power semiconductor switch.
2. Description of the Related Art
A power semiconductor switch requires a so-called driver. The driver has a primary side and a secondary side separated from the primary side with regard to its electrical potential. On the primary side, a signal voltage in the form of a switch-on/off signal for the power semiconductor switch is fed into the driver, e.g., by a standard 5 V logic circuit having, e.g., the installation ground as reference potential together with the primary side.
With their reference potential, the secondary side and thus the semiconductor switch, are at the potential of the power semiconductor switch, e.g., 10 V above the installation ground. During the switching operation of the semiconductor switch, in particular, the reference potential of the secondary side and of the semiconductor switch can fluctuate greatly upward or downward. Consequently, there is a potential difference between the primary side and the secondary side. In the case of power semiconductor switches of low power classes which are used in forklift trucks, for example, the maximum potential difference between the primary side and the secondary side is in this case around 200 V.
For directly driving the semiconductor switch, a more powerful and switching voltage is available on the secondary side than is available on the primary side. The switching signal or the switching voltage therefore has to be transmitted, and if appropriately amplified or modified, within the driver from the primary side to the secondary side, that is to say across the potential limit. Various solutions are known for this purpose.
The direct transmission of the driving power by a power pulse by means of a transformer is known, that is to say that the switching signal per se is transmitted from the primary side to the secondary side by means of a transformer. In this case, the secondary-side switching input, e.g., the gate of the power semiconductor switch, is connected, e.g., directly to the transformer output.
The transformer-based transmission of driving pulses in conjunction with a secondary-side edge storage unit is also known. This involves a pulse having a different polarity being generated from a primary-side logic signal having the switching states Hi and Lo, e.g., by means of a capacitor as differentiator for each state change of the signal (Hi-Lo or Lo-Hi). The pulse is transmitted from the primary side to the secondary side by means of an RF transformer. Situated on the secondary side there is an edge storage unit, e.g., in the form of a flip-flop, which is set by pulses of one polarity and cleared by those of the other polarity, and thus provides a logic signal corresponding to the input signal. In this case, therefore, rather than the switching signal per se, only the state change of that signal is transmitted by means of the transformer and the switching state, that is to say on or off, of the power semiconductor switch is stored in the secondary-side edge storage unit.
The capacitive transmission of pulses with an output-side edge storage unit is also known. This involves a current pulse having a different polarity being generated from the abovementioned logic signal for the respective state change thereof. The pulse is transmitted from the primary side to the secondary side by means of the capacitor. The edge storage unit mentioned is driven in a manner corresponding to that above by means of the current pulses or the respective voltage drop generated at a resistor by said pulses.
Finally, the transmission of voltage and/or current pulses with a secondary-side flip-flop storage unit is known, which involves generating a current pulse from the abovementioned logic signal, which current pulse is transmitted to the secondary side by means of a high-voltage transistor and again drives, e.g., the above-mentioned flip-flop there.
It is also known to code switching pulses, e.g., with the aid of a frequency coding, and to decode them on the secondary or output side. In this case, pulses having, e.g., a different frequency are generated from the logic signal mentioned, depending on the logic state or state change of the signal, by means of a coding unit. The transmission of the pulses to the output side is once again effected by means of an RF transformer. A decoding logic is then once again situated on the secondary side, which decoding logic is set and reset by the pulses having, e.g., a different frequency and thus once again provides the above-mentioned output signal.
The known solutions all have disadvantages. Transformer-based forms of transmission require large inductive components that are usually afflicted with large value tolerances. What is common to all of the pulse transmission methods is that an erroneous transmission of a pulse, e.g., as a result of interference during the pulse transmission, can lead to an incorrect switching state of the power semiconductor switch for the entire clock duration, that is to say until the next pulse arrives. Secondary-side state storage units, such as, e.g., edge storage units or flip-flops, always involve the risk that, as a result of brief interference, both during the driving of such state storage units and in the state storage unit itself, a longer-term erroneous switching state of the power semiconductor switch arises until the state storage unit is switched into the correct switching state again by a correct pulse or correct information.